The invention relates to a novel CRT (cathode-ray tube) comprising a beaded electron gun mount assembly disposed in a glass neck of the tube in which the insulating support beads of the electron gun carry electrically-conductive coatings for suppressing arcing therein; and more particularly for suppressing flashovers in the neck of the CRT. The electrically-conductive coatings are of a size and are located so as to permit electrical processing of the tube without adverse effects.
A color television picture tube is a CRT which comprises an evacuated glass envelope including a viewing window which carries a luminescent viewing screen, and a glass neck which houses an electron gun mount assembly for producing one or more electron beams for selectively scanning the viewing screen. Each gun comprises a cathode and a plurality of electrodes supported as a unit in spaced tandem relation from at least two elongated, longitudinally-oriented support rods, which are usually in the form of glass beads. The beads have extended surfaces closely spaced from and facing the inner surface of the glass neck. The beads usually extend from the region close to the stem, where the ambient electric fields are small, to the region of the electrode to which the highest operating potential is applied, where the ambient electric fields are high during the operation of the tube. The spaces between the beads and the neck surfaces are channels in which leakage currents may travel from the stem region up to the region of the highest-potential electrode. These leakage currents are associated with blue glow in the neck glass, with charging of the neck surface and with arcing or flashover in the neck.
Several expedients have been suggested for blocking or reducing these leakage currents. Coatings on the neck glass are partially effective to prevent arcing but are burned through when arcing does occur. A metal wire or ribbon in the channel (partially or completely around the mount assembly) is also partially effective to reduce arcing because it is often bypassed due to its limited longitudinal extent, because the limited space between the bead and the neck may result in shorting problems, and because there is frequently field emission from the metal structure.
One other expedient which has been found to be particularly effective is disclosed in U.S. Pat. No. 4,288,719 to K. G. Hernqvist issued Sept. 8, 1981. The Hernqvist patent discloses a CRT including a beaded electron-gun mount assembly in which each glass bead has a rectangularly-shaped electrically-conductive metal coating on the bead surface facing the neck. It has been found, however, that when the electrodes of the electron gun are electrically processed; e.g., by spot-knocking,the electrically-conductive coatings are eroded, producing undesirable particles in the CRT. Spot-knocking is described in U.S. Pat. No. 4,214,798 to Hopen issued on July 29, 1980.
Yet another expedient which has been effective is described in U.S. Pat. No. 4,567,400 to S. A. Opresko issued Jan. 28, 1986. The Opresko patent discloses that the electrically-conductive coatings should be positioned opposite a focusing electrode and spaced a prescribed distance away from a gap between the end or anode electrode and the adjacnet focusing electrode. Additionally, no portion of the electrically-conductive coatings should be opposite a claw on the focusing electrode. However, the coatings described in the Opresko patent adversely affect not only the degree of spot-knocking activity, i.e., the number of induced electrical discharges but also the region of the electron gun mount assembly in which the discharges take place. In particular, the electrical activity during the spot-knocking process is typically seven times higher for CRT's with the electrically-conductive coatings on the insulating beads than for CRT's without the coatings. This high degree of electrical activity is known to generate bead, stem, and glass-neck particles which may cause blocked apertures in the shadow mask of the tube. In addition, the prior electrically-conducting coatings concentrate the spot-knocking activity in the low voltage region of the mount assembly. Thus, the spot-knocking activity in the high voltage region of the mount (between the anode electrode and the focusing electrode) is reduced and the subsequent high voltage characteristics, i.e., leakage currents and afterglow are not optimized.